The present invention relates to a web handling apparatus and particularly relates to an oscillating pinch roll assembly utilized in conjunction with the extrusion of blown plastic films.
In the production of plastic films, the roll formation or the characteristic shape of a cylindrical roll of the film is a very important characteristic indicative of the overall quality of the film. This is especially true with soft and light gauge films that are commercially sold in a roll form.
A perfect or near perfect cylindrically shaped roll is a mandatory requirement for many plastic film applications and uses. A perfect cylindrical shaped roll implies a perfect uniform gauge across the plastic film or web. This uniform gauge is very difficult to obtain in films made by a casting or made by a calendering process. In the extrusion of plastic films, it is virtually impossible to obtain a uniform film gauge that will, in turn, produce a uniform, cylindrical roll. This is especially true due to the nature of the gauge variations in the web (i.e., the film) which are very localized in the cross machine direction. In other words, one can identify narrow sections in the thickness or gauge of the film approaching or exceeding the high gauge tolerance as well as web sections approaching or exceeding the low gauge tolerances of the film. In commercially sized, cylindrical rolls that normally include thousands of layers of film, these high and low gauge sections add up algebraically thereby deforming the outer contour of the cylindrical roll; hence, "high bands" and "valleys" are readily identified in the roll. This condition affects the flatness of the web when the film is unrolled.
The introduction of extrusion blown films, where a thin wall tubing is formed by extension, improved the production of cylindrical rolls of film having "good" profiles, that is perfect or nearly perfect cylindrical shapes, without "bands" and "valleys". Production of these good profile rolls was possible by distributing the gauge tolerances laterally across the roll or laterally distributing the imperfection along a longitudinal run of the web.
In this process of extruding blown films, most of the gauge variations originate between the extruder die and the "frostline", which is an area approximately located near where the plastic bubble becomes cylindrical. In general, plastic is extruded through a generally circular die and an area of high air pressure is maintained in the "bubble" of plastic. The plastic is drawn up from the extruder by a pinch roller mechanism and the air pressure in the bubble is maintained constant. The extruded bubble of plastic is formed into a tubular web and the tubular web is drawn between two pinch rolls to obtain the double walled, lay flat.
After the tubular web or tip of the bubble is collapsed and flattened, the resulting double walled web can be wound as a "lay flat", can be slit on both edges and wound on two rolls each carrying a single layer of film, or can be slit at one point and then opened to obtain one single layer roll having a width double that of the lay flat.
In a simple extrusion blown film assembly, any particular longitudinal line on the plastic bubble remains in the same lateral location on the finished roll of film as it was in the original tubular web. This longitudinal imperfection or defect produces a bad or uneven cylindrical roll due to the cumulative effect of this gauge variation.
By rotating the pinch rolls in a plane normal to the center line of the bubble (or tubular web) and about an axis that is conjointly the center line of the tubular web, the constant, longitudinal, imperfect gauge variation line in the bubble will change its relative position across the lateral width of the roll of film. If the pinch rolls are rotated slowly and at a uniform rate through a 360 degree angle, each and every line or imperfection in the bubble traverses the full lateral width of the web. This produces an even distribution of any imperfections in the film gauge across the web. For simplicity of power transmission, by reversing the rotation of the pinch rolls and counter rotating them 360 degrees in the opposite direction, in the same slow and uniform rate, the imperfections are further laterally distributed over the longitudinal run of the web. Therefore, by introducing an oscillatory rotation of the pinch rollers with respect to the bubble center line, any imperfections can be laterally distributed along the width of the cylindrical roll. Hence, a perfect cylindrical roll film is wound regardless of the gauge variations in the tubular web. The same result can be obtained by rotating the bubble and keeping the pinch rollers stationary.
From the early stages of development of this blown film extrusion process, this technique of rotating the pinch rollers with respect to the center line of the bubble, has been applied in many different ways to obtain the lateral distribution of film gauge variations over a longitudinal run of the film. For example, the winder and the pinch rolls have been oscillated about the bubble center line when the extruder is stationary, with the winder or the extruder on the vertically upper top of the extrusion apparatus. Another method is to rotate the extruder about the extrusion or bubble center line while keeping the pinch rollers and winder stationary. Vertical extruders mounted on a rotating base were developed to save floor space in the manufacturing facility. Also, rotational dies have been widely used to generate a rotation between the pinch rollers and the tubular web.
Most recently, wider films have been required by industry which in turn dictate the use of larger equipment. Further, with the introduction of co-extrusion processes that use more than one extruder and the need to extrude polymers having poor heat stability, it is more and more difficult to use the conventional methods of rotating the extruder or rotating the winder with pinch rolls attached, or otherwise rotating the die and the air ring in order to obtain a good or nearly perfect cylindrically finished roll.
One device was developed in which a stationary extruder, die and winder were used with only the pinch rollers being oscillated about the bubble center line. An idler roller was mounted with the pinch rollers parallel to the pinch rollers and positioned outwardly and slightly higher than the pinch rollers. A turn bar was horizontally located near and rotatable about the extrusion or bubble center line. This turn bar oscillated at one-half the angular speed of the pinch rolls. The trajectory of the lay flat over this idler roll and turn bar made a 180 degree turn. As a result, the output path of the web was stabilized by fixing it over one stationary idler roller and bringing the web down from the tower (upon which was located the pinch rollers and web handler) to the stationary winder. The oscillation of the pinch rollers was 180 degrees and this technique produced good results with a reasonably good gauge control.
This principle was used in a double version with two idler rollers and two turn bars in U.S. Pat. No. 3,768,949 such that the oscillation could be extended through a 360 degree angle.
Another patent, U.S. Pat. No. 3,657,974 by Hedrich, et al., discloses the use of a pinch roller mechanism having a turn bar coplanarly turning the flattened web and two deflecting rollers at one axial end of the turn bar. The axis of rotation of this pair of deflecting rollers is parallel to the center axis of the tubular web. A third withholding roller, having an axis of rotation parallel to the tubular web center axis, is disposed at the other axial end of the turn bar. The flattened and turned web runs between the first pair of deflecting rollers, over one roller of that pair of deflecting rollers and to a fixed deflecting roller. The pinch rollers, turning bar and three deflecting rollers are rotated 360 degrees such that the web still passes between the pair of deflecting rollers, but then passes over the other pair of deflecting rollers and is spaced from the turn bar by the third deflecting roller at the other axial end of the turning bar. The unsupported distance transversed by the web is significant, i.e., the distance between the pair of deflecting rollers and the stationary idler roller. Further if the oscillatory motion is at a constant speed, change in the speed of the web will not be constant because of the straight line distance between the pair of deflecting rollers and the fixed roller varies disproportionately as compared to the oscillatory speed.